In digital communication systems, digital data can be represented as analog signals by use of a modulator/demodulator (modem). The modem converts a series of binary voltage pulses into an analog signal by encoding the digital data onto a carrier radio frequency. The resulting signal occupies a certain spectrum of frequency centered about the carrier and may be propagated across a communication medium. One of the requirements of signal modulation is that no long and repeatable series of one's and zero's be present in the data stream. In the event of such repetition, a surge of power occurs near the radio frequency corresponding to the transmitted one's or zero's. This may result in a peak power to average power ratio higher than the current acceptable FCC standards.
Systems designed to prevent the transmission of continuous bits of one's and zero's are known as scramblers. Typical scrambling techniques that are currently in use provide a regenerative or recursive approach, in which previously transmitted data are used to decode the incoming data. However, a major disadvantage with the recursive type scrambler is that any single bit error that may occur during transmission would propagate and multiply into multiple bit errors. The result is an unacceptable reduction in the effect of any forward error correction employed by the communications system. Thus, there is a need for a scrambling/descrambling device and method which can effectively encode streams of data so that long repetitive patterns of zeros or ones can be prevented without disrupting the reception of data that may have occasional bit errors.
Accordingly, it is an object of present invention to prevent long streams of ones or zeros from appearing in the data stream applied to a radio frequency (RF) modulator of a transmitter.
It is a further object of the present invention to cause a permutation of the data in such a way that a series of continuous ones or zeros is transformed into a pseudo random sequence.
It is yet another object of the invention to provide a scrambler that does not propagate and multiply occasional single bit errors of an otherwise good data signal into multiple bit errors that may disrupt the flow of data.
It is still a further object of the invention to provide a descrambler which is located at the RF destination for reconstructing the permuted data back into the original data sequence.
The foregoing objects and advantages of the invention are illustrative of those which can be achieved by the present invention and are not intended to be exhaustive or limiting of the possible advantages which can be realized.